For processes that produce fatty acid methyl esters (FAME) for biodiesel fuel or other purposes, the predominant triglyceride feeds have been virgin refined vegetable oils and less expensive feeds such as waste cooking oils or yellow grease. Reactions to make the methyl or other alkyl esters typically follow the preferred base-catalyzed transesterification process. Such processes have been described in numerous publications including U.S. Pat. Nos. 2,494,366 and 4,695,411. If significant concentrations of free fatty acids are present in any feed to a base-catalyzed process, they will neutralize at least a portion of the base catalyst thereby wasting catalyst and potential product. The free acids are generally removed as part of the refining process for virgin vegetable oils creating a significant amount of soap stock. This constitutes a yield loss of material otherwise readily converted to FAME. In the case of waste oils and greases, these free fatty acids are commonly esterified with an alcohol, and the water of reaction must be removed to very low concentrations to drive the reversible reaction to nearly complete conversion of the acids. Otherwise, consumption of the base, typically sodium or potassium methoxide, will be excessive in the subsequent transesterification step as will be organic acid salt formation. For example, Kawahara et al in U.S. Pat. No. 4,164,506 discloses blowing methanol vapor into the reaction mix to compensate for methanol entrained out with the water of reaction.
The esterification of free fatty acids may be accomplished by operating in the presence of a mineral acid catalyst and often slightly higher temperatures than for transesterification. In U.S. Pat. No. 4,698,186, Jeromin et al discuss esterification processes employing either an acidic ion-exchange catalyst or toluene-sulfonic acid. Jackam et al in U.S. Patent Application 2007/0277432 teaches a method for converting the free fatty acids in triglyceride feedstocks utilizing glycerolysis of the fatty acids to glycerides which are then fed to a conventional transesterification step. Jackam discloses a reactive distillation step to recover and remove alcohol and glycerin from the FAME. U.S. Patent Application Publication 2007/0261294 teaches the use of a water immiscible alcohol and a very low concentration of sulfonic acid catalyst to accomplish the esterification such that very little of the subsequent base catalyst is consumed in neutralizing it.
A process for producing biodiesel from oleaginous seeds fed into a transesterification reactor is disclosed in U.S. Pat. No. 7,112,229. The are several notable features of this process: 1) the process is tailored toward castor bean feed and ethanol as the preferred alcohol, 2) the seeds are ground or chopped (communited) to a much smaller size, 3) the communited seeds are slurried only in alcohol initially, 4)transesterification with a basic catalyst is the only reaction step, hence no significant amount of free fatty acids from any source can be tolerated, and 5) distillation of the liquid crude product is necessary prior to glycerin decanting. These features present numerous drawbacks for the process of the U.S. Pat. No. 7,112,229 patent, although it does provide some evidence of the feasibility of starting with a seed feed in a biodiesel production process.
Wimmer in U.S. Pat. No. 5,434,279 discloses a process for reacting raw uncleaned oils or fats to a FAME using two steps of base-catalyzed transesterifcation with intermediate re-addition of separated glycerin. However, in the examples, the feeds contain at most 1.4 wt. percent free fatty acids, and since these feeds are subjected to a base-catalyzed reaction first, there cannot be any reaction of the free fatty acids to anything other than sodium or potassium salts of these acids. Hence, there is no yield advantage realized from the contained free fatty acids. In U.S. Pat. No. 7,109,363, Brunner discloses a process for production of fatty acid esters wherein a raw or waste oil undergoes an esterification with a water-free alcohol, followed by contact with a crude glycerin stream recycled from a downstream separation, and, following a phase separation, base catalyzed transesterification is effected, followed by another phase separation. Brunner does not acknowledge or disclose the disposition of the water of reaction that is necessarily formed in the esterification of free fatty acids. Furthermore, the Brunner process has the disadvantage of two steps in between the esterification and transesterification process.